Mechanical alloying is a powder metallurgy process consisting of repeatedly welding, fracturing and rewelding powder particles through high energy mechanical milling. Mechanochemical processing is the application of mechanical alloying techniques to induce chemical reactions and chemical refinement processes through sold state reactions. The energy of impact of the milling media, the balls in a ball mill for example, on the reactants is effectively substituted for high temperature so that solid state reactions can be carried out at room temperature.
Titanium and its alloys are attractive materials for use in aerospace and terrestrial systems. There are impediments, however, to wide spread use of titanium based materials in, for example, the cost conscious automobile industry. The titanium based materials that are commercially available now and conventional techniques for fabricating components that use these materials are very expensive. Titanium powder metallurgy offers a cost effective alternative for the manufacture of titanium components if low cost titanium powder and titanium alloy powders were available. The use of titanium and its alloys will increase significantly if they can be inexpensively produced in powder form.
Currently, titanium powder and titanium alloy powders are produced by reducing titanium chloride to titanium through the Kroll or Hunter processes and hydrogenating, crushing and dehydrogenating the resulting ingot material (the HDH process). The cost of production by these processes, particularly the HDH process, is much higher than is desirable for most commercial uses of titanium powders. In the case of titanium alloy powders, especially multi-component alloys and intermetallics, the cost of HDH production escalates because the alloys must generally be melted and homogenized prior to HDH processing.
Conventional methods for producing titanium by reducing titanium chloride is a multi-step process. In the first step, titanium ore in the form of titanium oxide TiO.sub.2 is chlorinated to form TiCl.sub.4, as shown in Eq. 1. EQU TiO.sub.2 +2Cl.sub.2 (in the presence of carbon at high temperature).fwdarw.TiCl.sub.4 (1)
Then, as shown in Eq. 2, the titanium chloride is reduced by magnesium or sodium at high temperature, above 800.degree. C., to form titanium. EQU TiCl.sub.4 +2Mg.fwdarw.Ti+2MgCl.sub.2 (2)
Titanium is tightly bonded to oxygen. This factor in conjunction with the high temperature chlorination and reduction processes lead to high cost. Additionally, the sponge/fines products contain salts (NaCl or MgCI.sub.2, depending on the specific process used). These chloride salts are leached out to obtain sponge Ti with chloride salt contamination levels of about 1500 ppm. Even with intense leaching/vacuum distillation, remnant salt remains at a level of 150 ppm and above. The remnant salt can be removed by the ingot melting step in the HDH process. Leaving remnant salt in the powder degrades the mechanical properties of the titanium, particularly those properties such as fatigue (S-N) that are initiation related. For use in high integrity applications a salt free powder is needed. For less demanding applications, a minimization of the cost of the powder is required. Presently, manufacturers must choose between low cost sponge fines which lead to inferior properties or high priced powders.
Commercial pure titanium powders with chloride salt levels less than 10 ppm can be obtained by crushing hydrogenated ingot material followed by dehydrogenation (HDH) or by reacting TiO.sub.2 with fluorine salts and then reducing the fluorinated titanium with aluminum. As noted above, the HDH process is prohibitively expensive for most commercial uses of titanium. A number of attempts have been made in the past to reduce the cost of producing titanium sponge. These include continuous injection of titanium chloride into a molten alloy system consisting of titanium, zinc and magnesium, vapor phase reduction and aerosol reduction. Although cost reductions as high as 40% have been estimated for some of these techniques, a common feature of all of these processes is the use of high temperatures to reduce titanium chloride or titanium oxide. The direct reduction of TiO.sub.2 is being considered as one way to reduce the cost of producing of titanium. So far as the Applicants are aware, the only method for the direct reduction of the oxide presently available is a Russian process of metal hydride reduction (MHR) at a high temperature, about 1100.degree. C. The reduction reaction between titanium oxide and calcium hydride is shown in Eq. 3. EQU TiO.sub.2 +2CaH.sub.2 .fwdarw.Ti+2CaO+2H.sub.2 (3)
The Russian process produces chloride free Ti powder in a single step reaction. Eq. 3 also shows the possibility of forming TiH.sub.2 if the reaction can be carried out at lower temperatures where TiH.sub.2 is stable.